Increased storage capacity in magnetic recording has traditionally been addressed through improvements in the ability to store information on a particular storage disc with an increased areal density, e.g., decreasing the size of the optical spot formed by the laser light in a magnetooptical system. Until recently, these prior art approaches have been adequate for increasing the storage capacity of magnetic recording discs.
Recently, however, the areal density in magnetic recording technologies has reached 70 Gbit/in2 in certain products, and is increasing at a rate of approximately 600% per year. Data rates are approaching Gbit/s levels and are increasing at a rate of approximately 30–40% per year. An earlier perceived density limit of 40 Gbit/in2 has been surpassed in laboratory demonstrations. Densities higher than 100 Gbit/in2 have been demonstrated. In perpendicular recording technology, densities in the range of 100–500 Gbit/in2 have been targeted, and are considered feasible. These projections are generally based upon scaling assumptions and projected future technological advancements in the areas of read/write heads, recording media, channel electronics, tribological coatings, head-to-disc interface and narrow track servo capabilities.
An area of particular importance in magnetic recording is media noise suppression. At higher areal densities, smaller particles, or grains, are required to reduce the intrinsic media noise and obtain a higher signal-to-noise ratio in the readback data. In addition to reducing and scaling the media grain size, control of the magnetic grain isolation and uniformity and control of the crystallographic texturing are also used to suppress media noise. Achieving low noise media by scaling to a small grain size, however, is limited by thermal instabilities. Such thermal instabilities are exhibited when using grain diameters below approximately 8–10 nm, and may render today's commonly used cobalt-alloy based recording media unsuitable for archival data storage purposes.
It has been found that smaller, stable grains can be obtained from magnetically harder materials, such as tetragonal L10 phased iron-platinum (FePt) or cobalt-platinum (CoPt) compounds, or artificially layered materials, such as Co/Pt or Co/Pd multilayers. FePt and CoPt compounds are known for their high magnetocrystalline anisotropy and magnetic moment, properties that are desirable for high-density magnetic recording media. Other candidates for smaller, stable grain sizes are rare earth transition metal compounds, such as Co5Sm or Nd2Fe14B. However, it may be difficult to maintain these materials in a chemically stable state in hard disc media where minimal overcoat thickness is mandatory.
A common problem with high anisotropy materials, such as FePt and CoPt compounds, is their large coercivity, which can reach values on the order of 50,000 Oe. Such large coercivities far exceed the write field capabilities of today's magnetic recording heads. In an effort to alleviate the problem of large coercivity, vertical recording and thermally assisted recording schemes have been proposed.
Additionally, advances in recording media based on cobalt (Co) are reaching the limits of superparamagnetism, as the conventional scaling approach is to reduce the media grain surface area in proportion to the bit cell surface area. Traditionally, the loss in grain volume has been compensated for by increasing the magnetic anisotropy of the magnetic particles. However, as noted above, the write field capabilities of the recording head limits this approach. The extendibility of this approach also appears to be rather limited. Three times smaller grain diameters, from currently about 9 nm down to about 3 nm, and correspondingly about 10 times higher aerial densities, become feasible if writing to magnetically much harder materials can be accomplished. Chemically ordered FePt alloys (L10) are key candidates for future generation ultra high-density magnetic recording media, owing to their enhanced magnetic anisotropy and the potential for largely reduced thermally stable grain sizes. However, as previously noted, the chemical stability of these materials presents a problem in the proposed environment in which they are to be used.
Another logical approach to enhancing the aerial density in magnetic recording is to reduce the grain count per bit. On the recording media side, the grain size distribution needs to be trimmed below 10% (sigma over mean) in order to reach grain counts as low as 10 grains per bit, as required in recent Tbit/in2 perpendicular recording models. Current state-of-the-art. sputtered recording media have grain size distributions of approximately 25%, and it remains an open challenge whether the required improvements in grain size distribution can be obtained using physical, thin film sputtering processes.
Lithographically patterned media, also known as bit-patterning, may postpone the arrival of thermal instabilities in the recording media. Bit-patterning combines several hundred media grains into one single magnetic island, which does not require large coercivities. A comprehensive review of such lithographically patterned media techniques can be found in G. Hughes, “Patterned Media” in Physics of Ultrahigh Density Magnetic Recording, chapter 7, ed. Plumer, van Ek, Weller, Springer (2001), which reference is hereby incorporated herein by reference. The achievable densities using this bit-patterning lithographic approach is limited by lithography to approximately 1 Tbit/in2. In order to push beyond the density limit set by lithography, self-assembled nanoparticle arrays have been proposed. These self-assembled, ordered and uniform nano-magnet arrays provide conceivable solutions to many proposed future recording schemes, e.g., conventional granular media, perpendicular media, thermally assisted recording and patterned media recording schemes. A representative 6 nm FePt nanoparticle array with typical dimensions and achievable areal densities is shown in FIG. 1.
The specific array 10 shown in FIG. 1 has a surface area of approximately 130×130 nm2 and includes approximately 260 particles per layer. The array 10 has a corresponding particle density per surface area of approximately 10 Tparticles/in2. In future single particle per bit patterned recording schemes, this particle density per surface area could lead to respective areal bit densities of approximately 10 Tbit/in2. However, the use of self-assembled nanoparticle arrays presents many technological challenges.
Colloidal chemistry synthesis and self-assembly procedures have demonstrated great promise in fabricating monodispersed magnetic nanoparticles in large area arrays. Self Organized Magnetic Arrays (SOMAs) may evolve into prospective alternative future ultra-high density magnetic recording media with an areal density potential far beyond 1 Tbit/in2. Fabrication of respective nanostructures with control over size, size distribution and chemical composition, however, remains a major challenge in the formation of nanoparticle arrays.
Whether these self organized magnetic nanoparticle arrays will become practical depends on numerous factors, but one of the main challenges involves the large scale ordering of particles, namely, how to pattern magnetic nanoparticles into organized assemblies on the surface of a disc substrate. As used herein, patterning means the 2-dimensional or 3-dimensional placement and registration of nanoparticles on a substrate with long-range ordering over a large area. Typical FePt arrays have been shown to order on a lateral structural coherence length, (i.e., the distance over which uniform, ordered arrays as shown in FIG. 1 can be formed), on the order of 100–1000 nm. This is far less than required in typical magnetic recording discs, which are approximately 3″ in diameter.
The present invention is directed toward overcoming one or more of the above-mentioned problems.